Biosynthesis of pteridines. Stopped-flow kinetic analysis of CTP cyclohydrolase I

GTP cyclohydrolase I catalyzes a mechanistically complex ring expansion aff ording dihydroneopterin triphosphate from GTP. The inherently slow enzyme r eaction was studied under single turnover conditions monitored by multiwave length ultraviolet spectroscopy. The spectroscopic data array was subjected to singular value decomposition and thereby shown to comprise six signific ant linearly independent optical processes. The data were fitted to a model of six consecutive unimolecular reaction steps where the first was conside red to be reversible. The rate-limiting step was shown to occur rather late in the reaction sequence.